Type 2 diabetes is a common, debilitating disease affecting approximately 7% of the US population. A progressive decline in pancreatic islet 2-cell function, manifest as decreased insulin release, is a fundamental cause of type 2 diabetes, but the reasons for this decrease are not well understood. In addition to insulin secreting 2-cells, pancreatic islets contain an extensive capillary network to facilitate the release of insulin and other islet hormones into the blood. In diabetes, endothelial dysfunction affects several tissues and underlies the development of microvascular complications in organs such as the kidney and eye. In this proposal we will test the hypothesis that diabetic hyperglycemia-induced endothelial dysfunction in the islet contributes to impaired insulin release. We have demonstrated in our preliminary data that endothelial dysfunction, in the absence of hyperglycemia, is sufficient to cause impaired insulin release in transgenic mice that lack endothelial nitric oxide synthase (eNOS) and develop endothelial dysfunction. Additionally, we have shown that markers of endothelial dysfunction (decreased eNOS levels and increased oxidative/nitrative stress in endothelial cells) is present in islets of diabetic db/db mice as early as around one week after the onset of hyperglycemia. These data support our hypothesis and led us to proposed the following specific aims: Specific Aim 1. To determine whether endothelial dysfunction is sufficient to impair insulin release. eNOS-/- mice that develop endothelial dysfunction and C57BL/6J control mice that do not will be studied for up to 12 weeks. We will determine insulin secretion in vivo and in vitro and will determine the time course of the development of impaired insulin release. Specific Aim 2. To determine the time course of diabetes-induced endothelial dysfunction in the islet and the role of hyperglycemia in its development. We will first determine the time course of development of islet endothelial dysfunction by studying db/db mice prior to the onset of diabetes, after ~1 week of diabetes and after 8 weeks of diabetes. We will then prevent the development of hyperglycemia with phlorizin treatment to determine whether this can prevent islet endothelial dysfunction. Specific Aim 3. To determine whether preventing endothelial dysfunction can improve insulin release in db/db diabetic mice. We will prevent the development of endothelial dysfunction in db/db mice by breeding db/+ mice with transgenic mice that overexpress eNOS. We will then determine insulin release in vivo.